The United States will stop Turkish forces flying and developing its F-35 stealth jets if Ankara goes ahead with the purchase of a Russian air defense system, the U.S. envoy to NATO said on Tuesday.

Washington and its allies have urged fellow NATO member Ankara not to install the S-400 system, saying that would let the technology learn how to recognize the F-35s, which are built to avoid tracking by enemy radars and heat sensors.

But Turkish President Tayyip Erdogan vowed anew on Tuesday to press on with the S-400 purchase despite allies’ concerns.

“We will hopefully start to receive the S-400 systems we purchased from Russia next month,” Erdogan told members of his AK Party in parliament. “Turkey is not a country that needs to seek permission or bow to pressures. The S-400s are directly linked to our sovereignty and we will not take a step back.”

Turkey has said its S-400 deal with Russia is final, exacerbating a diplomatic rift with the United States already widening over conflicting strategy in Syria, Iran sanctions and the detention of U.S. consular staff.

“Everything indicates that Russia is going to deliver the system to Turkey and that will have consequences,” Kay Bailey Hutchison, the U.S. ambassador to NATO, said in Brussels.

“There will be a disassociation with the F-35 system, we cannot have the F-35 affected or destabilized by having this Russian system in the alliance,” she told reporters.

The United States says the jets, made by Lockheed Martin Corp., give NATO forces a number of technological advantages in the air, including the ability to disrupt enemy communications networks and navigation signals.

Turkey produces parts of the F-35s fuselage, landing gear and cockpit displays. Hutchison said Ankara was an important partner in that production but that security concerns about Russia were paramount.

“So many of us have tried to dissuade Turkey,” she said.

PROBLEMS WITH ALTERNATIVES

The United States offered Turkey the more expensive Patriot anti-missile defense system, and then with a discount, but there were issues with the U.S. ability to deliver the Patriots quickly. Turkey also says that NATO allies have not helped it during times of heightened security concerns, and it therefore had to seek alternatives, and Russia came into the picture.

Germany and the United States stationed Patriot surface-to-air missile batteries in Turkey on a temporary basis in 2013 but moved them out in 2015, citing demands on assets elsewhere.

Washington also warned Ankara that it will face U.S. sanctions over the agreement with Moscow, a move that could deal a significant blow to Turkey’s ailing economy and its defense industry.

Turkey has dismissed the U.S. warnings, saying it would take the necessary measures to avoid complications, and proposed to form a joint working group with Washington to assess concerns. It has said U.S. officials have yet to respond to the offer.

The Pentagon has already stopped training Turkish pilots on the jets.

Erdogan is expected to discuss the issue with U.S. President Donald Trump at the G20 summit in Japan later this week. One senior NATO diplomat said that was probably the last chance of finding a solution.

NATO defense ministers, who meet for two days in Brussels from Wednesday, are not planning to formally raise the issue, but there could be some diplomacy in informal meetings, diplomats said.

“It’s not over until its over, but so far Turkey has not appeared to retract from the sale,” Hutchison. “The consequences will occur, we don’t feel there’s a choice in that.”